1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process and, for example, to the image forming apparatus such as a copier, a printer, a facsimile or the like.
2. Related Background Art
In recent years, the image forming apparatuses using the electrophotographic process are making progress toward higher-speed, higher function and colorized print, and various kinds of printers, recopying machines or the like are put on the market.
Among them, the image forming apparatus of an inline system wherein a plurality of image forming sections (image forming stations) are arranged in tandem so as to form toner images of different colors and a transfer material borne on a transfer belt as transfer material conveying means, for example, such as a sheet of paper is sequentially conveyed to each image forming station and the toner images of different colors are superimposed and transferred on the transfer material is capable of forming a color image at higher speed and hence considered to be promising as a bread-and-butter product of the color printers from now on.
The apparatus of the inline system is classified into the direct transfer system for directly multi-transferring the toner images of different colors on the transfer material conveyed by the transfer belt as described above and also the intermediate transfer system for sequentially superimposing and primarily transferring the toner images of different colors onto an intermediate transfer member (an intermediate transfer belt) and then for collectively and secondarily transferring them onto the transfer material. However, to realize miniaturization and lower cost, it is advantageous to adapt the direct transfer system because of fewer components used for the apparatus.
Also, in recent years, in view of an advanced functional design of the printers, a diversity of size, thickness (basis weight) of the transfer material and sheet papers (media) usable such as rough sheet papers in addition to the necessity for a two-sided printing are more and more required. Also, it is desired that the environment where the printers are used is not limited to offices alone where air-conditioners are fully equipped, but that even in the environment such as personal offices and own homes, an output image of good quality should be obtained because of the prevalence of SOHO (small office home office).
Thus, the printers, copying machines or the like are required to have increasingly advanced functions in view of media flexibility and using environment.
However, in the case of the inline system apparatus, since the configuration is such that a transferring is performed four times on the transfer material to form a full color image by using the transfer material and the transfer belt having unsteady factors in the value of resistance due to a change in the environment (temperature, humidity), there are some cases where an image fault occurs depending on a change in the environment wherein the apparatus is placed and the type of the transfer material used.
The transfer belt is a film member where electronic conductive agent such as carbon black and ion conductive agent is added on resin for regulating resistance. In the case of the electronic conductive agent, the value of resistance becomes uneven due to dispersion fault at the time of production, while in the case of the ion conductive agent, a water content contained in the transfer belt fluctuates due to a variation in the environment to cause a variation in the value of resistance.
On the other hand, the chief ingredient of the sheet paper is a highly hygroscopic cellulose and the value of resistance largely changes depending on its hygroscopic state. For example, in a high temperature and high humidity environment (H/H environment (30xc2x0 C./80%RH)) where the sheet paper absorbs the moisture, the resistance of the sheet paper is lowered to around 106 xcexa9cm, thereby making a given charge easy to leak, while in low temperature and low humidity environment (L/L environment) (15xc2x0 C./10%RH)), the value of resistance of the sheet paper is raised to around 1012 xcexa9cm, thereby making a charge injection hard to take place and making a charge difficult to induce.
Also, in the image forming apparatus having an automatic duplex mechanism for forming an image on two sides of the transfer material, when a toner image transferred on one side of the transfer material is fixed by a fixing device, the moisture inside the transfer material is evaporated and, at a step of transferring the toner image on the other side of the transfer material, the resistance of the transfer material is in a very high state.
In this manner, when the transferring of the toner image is performed against the transfer material such as the transfer belt having fluctuating factors of resistance, sheet paper or the like, a transfer fault occurs since a transfer current is hard to flow where the resistance is in a high state. On the contrary, when the resistance is in a low state, the transfer current flows excessively, thereby posing a problem in that the toner image transferred from a photosensitive member receives a reverse charge by an electric discharge and reverses in polarity so that it is transferred again on the photosensitive member with a transfer efficiency lowered.
Also, in the transfer portion of each image forming station, there is a problem in that the charge injection is hard to take place in the L/L environment where the resistance of the transfer material increases or when the automatic duplex mechanism is in operation and thus the transfer fault is easy to occur.
The problem similar to the transfer fault where the transfer belt has the fluctuating factors of resistance occurs also in the image forming station wherein the intermediate transfer belt having the fluctuating factors of resistance similar to the above described transfer belt is arranged along a plurality of the image forming stations and the toner images of different colors are sequentially superimposed and primarily transferred onto the intermediate transfer belt and then collectively and secondarily transferred onto the transfer material.
An object of the present invention is to provide an image forming apparatus, in which toner images on a first image bearing member and a second image bearing member are satisfactorily and sequentially transferred to a transfer material borne by a transfer material bearing member so that images of non-irregular color can be formed.
Another object of the present invention is to provide an image forming apparatus, in which toner images on a first image bearing member and a second image bearing member are satisfactorily and sequentially transferred to an intermediate transfer member so that images of non-irregular color can be formed.
Still another object of the present invention will become apparent by reading the following detailed description with reference to the accompanying drawings.